In a conventional cannon, a projectile is accelerated by the rapid expansion of gases resulting from the explosive combustion of propellant chemicals. The muzzle velocity of a projectile shot from a cannon is generally only slightly greater than the initial acoustic velocity of the expanding gases. This limitation results because the ballistic efficiency of the chemical propellant charge decreases rapidly as the driving gas expends most of its energy in accelerating itself. As used in the preceding statement, the term "ballistic efficiency" may be defined as the ratio of the rate of change of kinetic energy of the projectile to the rate of expenditure of chemical energy. It will thus be apparent that the decreasing ballistic efficiency inherently limits the acceleration of a projectile through the bore of a conventional cannon.
To overcome the limitation on projectile velocity imposed by driver gasdynamics, a new method for accelerating projectiles has been developed that does not use an exploding propellant charge, but instead continuously burns a combustible gaseous mixture to continuously accelerate a projectile in a method referred to as "ram acceleration." The new method is based on principles similar to those used in the air breathing ram jet engine, but is substantially different in many respects. For example, a ram jet engine carries with it a supply of fuel; in comparison, the projectile in a ram accelerator does not carry any propellant. Instead, the projectile travels through a tube filled with a mixture of gaseous fuel and an oxidizer compressed to several atmospheres of pressure. The tube functions like the outer cowling of a ram jet, and the profile of the projectile has a shape much like the center body of a ram jet. As the projectile passes through the combustible mixture, the gaseous mixture flows past the throat, i.e., the largest diameter portion of the projectile, into a diffusion area disposed immediately behind the throat, and burns in a combustion zone proximate the aft portion of the projectile. Combustion of the gaseous fuel proceeds in a forward moving combustion zone, producing an increased pressure which tends to accelerate the projectile down the bore of the tube. The ballistic efficiency may be maintained at a high level by tailoring the gas mixture in the tube so as to keep the projectile Mach number within prescribed limits.
At least five modes of ram acceleration are theoretically possible in the ram accelerator, depending upon the profile of the projectile, its velocity, and other factors. In two of the modes, combustion proceeds at subsonic velocities; in three other modes, a detonation wave attaches to the projectile. One of the subsonic combustion modes is referred to as a "thermally choked mode." A projectile can be accelerated in this mode to supersonic velocities in the range from approximately 0.7 kilometers per second to about 3.0 kilometers per second, i.e., the lower end of the range of velocities, which may be achieved in a ram accelerator. The thermally choked mode may be used to accelerate the projectile in a first portion of the tube, followed by a transition to one of the detonation modes further along the tube, by altering the composition of the gas mixture along the path of the projectile. Muzzle velocities in excess of 12 kilometers per second may thus be achieved. Details of the ram accelerator and of the modes of operation are described in U.S. patent application, Ser. No. 946,439, entitled "Apparatus and Method for Acceleration of Projectiles to Hypervelocities," filed Dec. 23, 1986, which is a continuation of application Ser. No. 623,829, filed June 22, 1984 (abandoned).
Early attempts to operate a laboratory test prototype ram accelerator in the thermally choked mode were unsuccessful. To preaccelerate the projectile to the supersonic velocity (approximately 700 meters per second) required to initiate ram acceleration in the test ram accelerator, compressed helium gas was applied behind a solid plug or sabot. The compressed gas propelled the sabot and the projectile placed in front of it through an evacuated portion of the tube. Once the projectile penetrated the portion of the tube filled with the combustible gas mixture, an on board igniter was provided, which was intended to immediately ignite the mixture, ram accelerating the projectile down the tube in the thermally choked mode. Inevitably, in each of the early attempts to initiate ram acceleration, there was a delay in the ignition of the combustible mixture, and then once ignited, a combustion driven wave rapidly propagated down the tube past the projectile, "unstarting" the flow, i.e., choking the flow past the projectile. In fact, the combustion wave proceeded to ignite the combustible mixture throughout the entire length of the tube, well ahead of the projectile, making it impossible to accelerate the projectile as hoped.
Further testing and research showed that the flow past the projectile was remaining supersonic, and that the supersonic flow was preventing a stable subsonic, thermally choked flow from being established as required. The problem was traced to the fact that it is extremely difficult to ignite and sustain combustion in a supersonic flow.
In consideration of the problems described above, it is an object of this invention to enable stable operation of a ram accelerator in a thermally choked subsonic combustion mode.
A further object is to establish a subsonic flow in the diffusion area behind the throat of a projectile as it initially enters a combustible gas mixture in a ram accelerator.
Yet another object of this invention is to initiate a stable subsonic combustion zone proximate an aft portion of the projectile.
These and other objects and advantages of the invention will be apparent from the attached drawings and the description of the preferred embodiments which follow.